1. Field of the Invention
The present invention relates to a method of calibrating sensor, and more particularly to calibrate the drift signal that is outputted from the potentiometric sensors during a long-time measuring.
2. Description of the Prior Art
The direct measurement of the hydrogen ion activity (pH) of the aqueous solution, by means of the glass membrane electrode, has been a valuable technology in analytical chemistry and process monitoring for many years. Due to wet storage, fragility, big size and high cost, the solid-state electrodes have been developed to substitute for the glass electrode.
In 1970, the first ion-sensitive field-effect transistor (ISFET) was fabricated by Bergveld (please refer to reference [1]: P. Bergveld, entitled “Development of an ion-sensitive solid state device for neurophysiological measurements”, IEEE Transactions on Bio-medical Engineering, vol. BME-17, pp. 70-71, 1970.). There are some advantages such as miniaturization, high input impedance and fast response of the ISFET. Furthermore, based on fast response, dry storage and low cost, the metal oxide electrode was investigated to substitute for the glass electrode by Fog and Buck (please refer to reference [2]: A. Fog, R. P. Buck, entitled “Electronic semiconducting oxides as pH sensors”, Sensors and Actuators, vol. 5, pp. 137-146, 1984.)
In the progress of technology, requirements of environment monitoring, home care, and automation industry stimulate the development of sensors. Biosensors are especially employed on clinical diagnosis. The drift, which is the inherent drawback of the sensor, results in the shift of the output signal of the sensor with time under the solution of constant composition and temperature.
For long-term measurement, the drift causes unstable signal and handicaps the wide application of sensors especially in the environment, which needs high accuracy (the corresponding reference [3] is published by L. Bousse, D. Hafeman, N. Tran, entitled “Time-dependence of the chemical response of silicon nitride surfaces”, Sensors and Actuators B, vol. 1, pp. 361-367, 1990., and further articles can be referred, for example, reference [4]: D. Yu, Y.-D. Wei, G.-H. Wang, “Time-dependent response characteristics of pH-sensitive ISFET”, Sensors and Acruators B, vol. 3, pp. 279-285, 1991., reference [5]: A. Garde, J. Alderman, W. Lane, “Improving the drift and hysteresis of the Si3N4 pH response using RTP techniques”, Sensors and Materials, vol. 9, pp. 15-23, 1997., reference [6]: P. Woias, L. Meixner, P. Frostl, “Slow pH response effects of silicon nitride ISFET sensors”, Sensors and Actuators B, vol. 48, pp. 501-504, 1998., reference [7]: S. Jamasb, S. Collins, R. L. Smith, “A physical model for drift in pH ISFETs”, Sensors and Actuators B, vol. 49, pp. 146-155, 1998., reference [8]: J. Hendrikse, W. Olthuis, P. Bergveld, “A method of reducing oxygen induced drift in iridium oxide pH sensors”, Sensors and Actuators B, vol. 53, pp. 97-103, 1998., reference [9]: R. Kuhnhold, H. Ryssel, “Modeling the pH response of silicon nitride ISFET devices”, Sensors and Actuators B, vol. 68, pp. 307-312, 2000., reference [10]: J.-C. Chou, C.-N. Hsiao, “Drift behavior of ISFETs with a-Si: H-SiO2 gate insulator”, Materials Chemistry and Physics, vol. 63, pp. 270-273, 2000., reference [11]: S. Jamasb, entitled “An analytical technique for counteracting drift in ion-sensitive field effect transistors (ISFETs)”, IEEE Sensors Journal, vol. 4, pp. 795-801, 2004.). The present invention provides a method of the drift calibration and the method is performed on the circuit to improve the long-term stability of potentiometric sensors.
Some prior arts disclosed these techniques, in United States patent, U.S. Pat. No. 4,701,253 to Hendrikus C. G. Ligtenberg, Jozef G. M. Leuveld, Date of patent: Oct. 20, 1987, entitled “ISFET-Based measuring device and method for correcting drift” provided an apparatus of the drift calibration for ISFETs. More particularly, the measuring device comprises an ISFET used as a chemically selective ion sensor, a reference electrode positioned adjacent the ISFET, an amplifier coupled to ISFET and control/correction circuitry coupled to ISFET, to the reference electrode and to the amplifier. The control/correction circuitry is operable to maintain the drain-source current IDS of the ISFET at a constant value and to correct drift effect of the ISFET on the basis of the logarithmic equation: ΔVp=A ln(t/t0+1), where: ΔVP is potential drift, A means scale factor for drift and amplitude, t0 is time constant defining the dependence on time, t indicates the time during which the sensor is operative in the event of continuous operation.
Another prior art disclosed by Hendrik H. v. d. Vlekkert, Nicolass F. de Rooy in the United States patent, U.S. Pat. No. 4,691,167, Date of patent: Sep. 1, 1987, entitled “Apparatus for determining the activity of an ion (pIon) in a liquid”. The patent provided an apparatus for determining the activity of an ion. The device comprises a measuring circuit including an ISFET, a reference electrode adjacent the ISFET, a temperature sensor, and control, computing and memory circuits coupled to the amplifiers and operable to maintain two of the following three parameters, Vgs (gate-source potential), Vds (drain-source potential) and ID (drain-source current) at a constant value so that the third parameter can be used for determining the ion activity or pIon. The pIon sensitivity of the apparatus, as a function of temperature and/or the variation of the drain-source current, ID, as a function of the temperature are controlled by controlling the Vgs so that the pIon can be calculated from a formula stored in the memory.
Furthermore, Avron I. Bryan, Michael R. Cushman disclosed the relating technology in the United States patent, U.S. Pat. No. 5,046,028, Date of patent: Sep. 3, 1991, in titled of “System for calibrating, monitoring and reporting the status of a pH sensor”. They provided a device for use in a system for providing on-line, real-time monitoring of the condition of a sensor immersed in a process solution. The sensor has a membrane and knows changes in characteristics of the process solution adjacent the membrane by periodically generating. The devices provide a fixed volume of the process solution adjacent the membrane and the fixed volume is independent of a flow rate of the process solution. The devices include a shield of non-conductive material, and have a retracted position permitting flow of the process solution past membrane to provide a fixed, stationary volume of process solution adjacent said membrane. The shield is selectively moved from its retraced position to its extended position during monitoring of the condition of the sensor.
Katsuhiko Tomita, Tsuyoshi Nakanishi, Syuji Takamatsu, Satoshi Nomura, Hiroki Tanabe, United States patent, U.S. Pat. No. 5,814,280, Date of patent: Sep. 29, 1998, in titled of “Semiconductor pH sensor and circuit and method of making same” provided a method to make pH sensor and circuit on the same substrate. A pH sensor having an ISFET is provided on a crystalline substrate of silicon with a thin film of aluminum oxide formed to have epitaxial growth with an overlaying thin film of silicon epitaxial grown on the aluminum oxide layer. A source element and a drain element are provided on the silicon film, and a pH responsive film layer is connected to the source and drain. The pH sensor can be accompanied with appropriate circuitry also integrally formed on the same epitaxial SOI substrate.
Please also refer to United States patent, U.S. Pat. No. 6,464,940, Date of patent: Oct. 15, 2002 to Koji Akioka, Akira Sanjoh, entitled “pH sensor and pH measurement method employing the same”. The prior art provided a pH sensor, which is capable of readily determining the pH of a solution of a small amount. The pH sensor includes a semiconductor substrate, an oxide film provided on the semiconductor substrate, a solution storage part for holding a solution on the oxide film, and an electrode to be in contact with the solution in a vicinity of the oxide film. To determine the pH of a solution, a capacitance-voltage characteristic is initially monitored by the sensor between the electrode in contact with the solution and another electrode provided on the back surface of the semiconductor. Then the pH of the solution is derived from a flat band voltage, which is obtained based on the capacitance-voltage characteristic.
Torsten Poechstein, United States patent, U.S. Pat. No. 6,624,637′ Date of patent: Sep. 23, 2003, in titled of “Device for measuring the concentrations in a measuring liquid” provided a device for measuring the concentration of hydrogen ions. The invention relates to a device for measuring the concentration of ions, notably of hydrogen ions, in a measuring liquid using at least one ion-sensitive field effect transistor which is integrated into an electric circuit within the device in such a way that said circuit emits an output signal which serves as measurement of the ion concentration in the measuring liquid. To provide a circuit which is as simple as possible, the invention provides for the at least one pH-ISFET to be bridge-connected with at least three resistors.